1. Field of the Invention
The invention relates generally to tunneling magnetoresistance (TMR) devices, and more particularly to a TMR read head with a magnesium oxide (MgO) tunneling barrier layer.
2. Description of the Related Art
A tunneling magnetoresistance (TMR) device, also called a magnetic tunneling junction (MTJ) device, is comprised of two ferromagnetic layers separated by a thin insulating tunneling barrier layer. The barrier layer is typically made of a metallic oxide that is so sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the two ferromagnetic layers. While various metallic oxides, such as alumina (Al2O3) and titanium oxide (TiO2), have been proposed as the tunneling barrier material, the most promising material is crystalline magnesium oxide (MgO). The quantum-mechanical tunneling process is electron spin dependent, which means that an electrical resistance measured when applying a sense current across the junction depends on the spin-dependent electronic properties of the ferromagnetic and barrier layers, and is a function of the relative orientation of the magnetizations of the two ferromagnetic layers. The magnetization of one of the ferromagnetic layers, called the reference layer, is fixed or pinned, while the magnetization of the other ferromagnetic layer, called the free layer, is free to rotate in response to external magnetic fields. The relative orientation of their magnetizations varies with the external magnetic field, thus resulting in change in the electrical resistance. The TMR device is usable as a memory cell in a nonvolatile magnetic random access memory (MRAM) array and as TMR read head in a magnetic recording disk drive.
FIG. 1 illustrates a cross-sectional view of a conventional TMR read head 10. The TMR read head 10 includes a bottom “fixed” or “pinned” reference ferromagnetic (FM) layer 18, an insulating tunneling barrier layer 20, and a top “free” FM layer 32. The TMR read head 10 has bottom and top nonmagnetic electrodes or leads 12, 14, respectively, with the bottom nonmagnetic electrode 12 being formed on a suitable substrate. The FM layer 18 is called the reference layer because its magnetization is prevented from rotation in the presence of an applied magnetic field in the desired range of interest for the TMR device, i.e., the magnetic field from a recorded region of the magnetic layer in a magnetic recording disk. The magnetization of the reference FM layer 18 can be fixed or pinned by being formed of a high-coercivity film or by being exchange-coupled to an antiferromagnetic (AF) “pinning” layer. The reference FM layer 18 may be part of an antiparallel (AP) pinned or flux-closure structure, where two ferromagnetic layers are separated by an antiparallel coupling (APC) spacer layer and thus antiparallel-coupled to form a flux closure, as described in U.S. Pat. No. 5,465,185. The magnetization of the free FM layer 32 is free to rotate in the presence of an applied magnetic field in the range of interest. In the absence of the applied magnetic field, the magnetizations of the FM layers 18 and 32 are aligned generally perpendicular in the TMR read head 10. The relative orientation of the magnetizations of the FM layers 18, 32 determines the electrical resistance of the TMR device.
TMR devices with MgO tunneling barriers, like CoFe/MgO/CoFe devices, exhibit a very large magnetoresistance due to coherent tunneling of the electrons of certain symmetry. However, MgO tunnel junctions are required to have (001) epitaxy and perfect crystallinity. The MgO barrier layer is typically formed by sputter deposition and subsequent annealing, which forms the crystalline structure. It has been found that when boron (B) is used in one or more of the reference and free ferromagnetic layers, higher tunneling magnetoresistance (ΔR/R or TMR) is observed after annealing. The amorphous CoFeB layer is known to promote high-quality crystallization of the MgO into the (001) direction, and thus higher TMR.
In a TMR read head, the free ferromagnetic layer should produce a high TMR and low magnetostriction. The free layer is typically a multilayer that includes a first ferromagnetic layer like CoFe or CoFeB near the MgO barrier layer that typically has high spin polarization but also high positive magnetostriction. To compensate for this the free multilayer also includes a relatively thick layer of NiFe alloy with negative magnetostriction and low Fe content, typically less than about 15 atomic percent (at. %), as a second ferromagnetic layer. However, the low-Fe NiFe second layer has a face-centered-cubic (fcc) crystalline structure, which destroys the epitaxial relationship between the MgO barrier and the first free layer after annealing. This leads to low TMR. To alleviate this problem, amorphous separation layers, like Ta, may be formed between the first and second layers. Ferromagnetic amorphous separation layers, like CoFeBTa, have also been proposed, as described in U.S. Pat. No. 8,427,791 B2, which is assigned to the same assignee as this application. However, these amorphous separation layers may result in a free layer with a high Gilbert damping constant (the parameter a, which is a dimensionless coefficient in the well-known Landau-Lifshitz-Gilbert equation). High damping results in high thermal-induced magnetic noise (sometimes called “mag-noise”). The effect of thermal excitations on the free layer becomes increasingly important as the free layer volume (and therefore its magnetic energy) is reduced. Because mag-noise is also proportional to the TMR signal, if the TMR is large then mag-noise is the dominant noise source in the TMR device and will limit the achievable signal-to-noise ratio (SNR). Thus it is desirable to design TMR devices with low damping so that mag-noise is suppressed.
What is needed is a TMR device with high TMR that has a MgO barrier layer and a free layer with low magnetostriction and low damping.